What is the Double-Checked Locking Pattern?

A concurrency design pattern used to reduce the overhead of acquiring a lock by testing the locking criterion before acquiring the lock.

It is primarily used to safely implement lazy initialization in multi-threaded environments (like a Thread-Safe Singleton). The first check avoids the expensive lock operation if the instance already exists. If not, the lock is acquired, and a second check is performed because another thread might have initialized it while waiting for the lock. The variable MUST be declared volatile to prevent CPU instruction reordering.